General Physics Question:

It deals with measurement of illuminating power and involves
measurement of the quantity of light.

Illuminating power of two sources of light is directly
proportional to the squares of the distances at which they
can produce equal intensities of illumination.

The instruments used for comparing the illuminating powers
of two sources of light are called photometers. They are.

1.Runford?s Shadow Photometer

2. Bunsen?s Grease-Spot Photometer

3. Flicker Photometer.